1. Field of the Invention
The present invention is directed to a method of utilizing distillation residues of phtalic acid anhydride or maleic acid anhydride.
2. Description of the Related Art
Phthalic anhydride (PA) is produced industrially by oxidation of o-xylene or naphthalene with air, in the gas phase. The PA produced in this process must be distilled in order to satisfy high purity requirements placed upon it. The distillation produces a residue which is a liquid at temperatures above c. 180.degree. C., and which yields a mass which is viscous to glassy when cooled.
Two methods of disposing of this residue are used industrially. In one such method, the distillation is carried out only to the point where the residue contains 50 wt. % PA. At high temperatures, the residue has relatively low viscosity, so that it can be pumped. It is fed to appropriate incineration devices and is incinerated. In order to maintain the residue in a state in which it can be readily conveyed, valuable products are left in the residue. Therefore, when the residue is incinerated, the valuable products are incinerated as well. This is a disadvantage of the first method.
In the second method, the PA is distilled off to the point where the residue is just barely capable of being removed from the distillation apparatus. During this removal, PA escapes, and can lead to contamination of the apparatus. The residue is cooled off in tank, and then must be broken up and transported to the incineration equipment. This processing is cumbersome and expensive.
A distillation residue is also produced in the manufacture of maleic anhydride (MA). Heretofore, this residue has been dissolved in water and discharged as waste water, which represented a substantial burden on the environment.
In view of the disadvantages of prior methods of treating the distillation residues of phthalic acid anhydride or maleic acid anhydride, a need continues to exist for new methods of processing these residues.